Traditional automotive vehicles include a powertrain that is comprised of an engine, a multi-speed transmission, and a differential or final drive system. The multi-speed transmission increases the overall operating range of the vehicle by permitting the engine to operate through its torque range a number of times. The number of forward speed ratios that are available in the transmission is indicative of the number of times the engine torque range is repeated.
Automatic transmissions (e.g., planetary gear type) and manual shifting transmissions (e.g., countershaft type) constitute the two main varieties of automotive transmissions. Early automatic transmissions had two speed ranges. This severely limited the overall speed range of the vehicle and therefore required a relatively large engine that could produce a wide speed and torque range. This resulted in the engine operating at a specific fuel consumption level during cruising, other than the most efficient level. Therefore, historically, manually-shifted transmissions were the most popular.
With the advent of three- and four-speed automatic transmissions, the automatic shifting transmission increased in popularity with the motoring public. These transmissions improved the operating performance and fuel economy of the vehicle. Increasing the number of speed ratios in an automatic transmission reduces the step size between each ratio, improving the shift quality of the transmission by making the ratio interchanges substantially imperceptible to the operator under normal vehicle acceleration. Furthermore, by varying the gear ratios within the transmission, the internal combustion engine may operate below the maximum rotational speed and preferably near the engine speed range that provides the best performance and/or fuel economy.
Automatic transmissions generally effect a ratio change by altering the power delivery path after selective actuation of one or more friction elements, commonly referred to as either a clutch or brake mechanism. The aforesaid friction elements may include, for example, a one-way clutch configured to selectively transfer torque in a first direction, and to interrupt the transfer of torque in the opposite direction. As an example, a one-way clutch may be implemented to transfer torque from an engine to the transmission, and to interrupt the transfer of reverse torque from the transmission to the engine.